sub add_son_father_pair {
  print "Enter a male name: ";
  chomp (my $name1 = lc <STDIN>);

  if (exists $son_father{$name1}) {
     print "Duplicate name -- try again!\n";
     return;
  }

  print "Add a father: ";
  chomp (my $add_dad = lc <STDIN>);
  $son_father{$name1} = $add_dad; 
}